Retroviral infections in sheep and goats: small ruminant lentiviruses and host interaction

Abstract

Small ruminant lentiviruses (SRLV) are members of the Retrovirus family comprising the closely related Visna/Maedi Virus (VMV) and the Caprine Arthritis-Encephalitis Virus (CAEV), which infect sheep and goats. Both infect cells of the monocyte/macrophage lineage and cause lifelong infections. Infection by VMV and CAEV can lead to Visna/Maedi (VM) and Caprine Arthritis-Encephalitis (CAE) respectively, slow progressive inflammatory diseases primarily affecting the lungs, nervous system, joints and mammary glands. VM and CAE are distributed worldwide and develop over a period of months or years, always leading to the death of the host, with the consequent economic and welfare implications. Currently, the control of VM and CAE relies on the control of transmission and culling of infected animals. However, there is evidence that host genetics play an important role in determining Susceptibility/Resistance to SRLV infection and disease progression, but little work has been performed in small ruminants. More research is necessary to understand the host-SRLV interaction.

Figure 1

Small Ruminant Lentivirus (SRLV) timeline. SRLVs are transmitted via the respiratory or lactogenic route. Proviral DNA transcription and gene expression is suppressed until infected monocytes mature into macrophages. Once in the target organs, infected macrophages initiate viral replication, which induces an inflammatory cascade. Despite a vigorous immune response, the virus is not cleared and the animal will remain a carrier for life. Most animals remain asymptomatic, but a certain percentage of the animals develop the clinical disease in one or several target organs. Subclinical infection and disease progression may range from months to years. Both Visna/Maedi (VM) and Caprine Arthritis-Encephalitis (CAE) are ultimately fatal. Host genetic factors, infecting viral strain and management influence the occurrence, length and spectrum of affected target organs.